Here are a few items you need to do to prepare your garden in January if you want to get an early start on your plantings:
Plan your garden – What do you want to grow? Are there any new varieties you want to try? Where are you going to get your plants?
Order your seeds if you plan on growing your vegetables from seed.
Have your soil tested and add any recommend amendments to the soil as you prepare it (plow in the additives like lime etc.). Your local county extension office can do the soil test for you.
If you use compost, now is the time to work it into your soil.
For those of you with limited space, now is the time to build any raised beds or square foot gardens if you plan on utilizing either this spring. For more on square foot gardening check out this book and this book review.